This invention relates to flexible film laminates and retortable food pouches fabricated therefrom.
In recent years, the food packaging industry has shown intense interest in the concept of pouch-packed foods which, among other advantages, do not require freezing for their preservation and can therefore dispense with costly and energy intensive refrigerated transportation and storage facilities. Much effort has gone into the development of a flexible retortable, or autoclavable, food pouch which cannot only withstand the rigors of sterilization and later reheating and provide barrier properties which are sufficient to adequately protect the contents during storage, but which employs materials that are toxicologically safe. The latter requirement necessarily narrows the range of useful materials, especially with regard to the adhesive used in laminating the layers of the pouch. The U.S. Food and Drug Administration (FDA) and its counterpart agencies around the world have established maximum levels of extractables which cannot be exceeded if a particular laminate is to be cleared for use in food packaging.
Among the various materials which have been investigated for use in the manufacture of the food pouch laminate layers are polyester, polyamide, polyolefin, especially polypropylene, ionomer resin, aluminum, and the like. Adhesives which have been studied for bonding the layers to each other include polyurethane, epoxy resin, ethylene-vinyl acetate copolymer and the maleic anhydride adduct of ethylene-propylene copolymer one of which, Morprime 78LJ10 (Morton Chemical, Division of Morton Norwich Products, Inc.), has been cleared for use in food packaging by the FDA and is commercially available.
In accordance with U.S. Pat. No. 3,453,173, a polyolefin-polyacrylonitrile laminate which is said to possess superior heat-seal strength and excellent barrier properties to the transmission of gases and is adaptable for the construction of food containers is prepared by bringing the polyolefin surface and the polyacrylonitrile surface together and adhering the adjacent surfaces to each other through an adhesive. Either or both surfaces can be pretreated in some manner to make them more adherent, e.g., by treatment with a gaseous mixture of boron trifluoride as described in British Pat. No. 834,196, by flame treatment and treatment by corona discharge as described in U.S. Pat. Nos. 2,632,921, 2,648,097 and 3,018,189 and British Pat. No. 986,680, or by treatment with a strong oxidant such as nitric acid. Among the adhesives employed in the manufacture of the laminate are ethylene-vinyl acetate copolymer described in U.S. Pat. No. 2,200,429, the rubber solutions described in U.S. Pat. No. 2,664,378 or the adhesive materials described in U.S. Pat. Nos. 2,994,632, 2,999,782, 3,049,455 and 3,179,554. While the laminate of U.S. Pat. No. 3,453,173 possesses properties which tend to make it a particularly worthwhile candidate for use in the construction of a food pouch, it has been observed that no, or at most very little, bonding of the polyolefin layer to the polyacrylonitrile layer is obtained with the FDA-approved resin Morprime 78 LJ 10 as the adhesive.
Another type of packaging laminate is described in U.S. Pat. No. 4,211,811 in which the surface of one or two stretched films made from substantially crystalline polyolefin is provided, on its surface in contact with the other film, with a metallized coating having a resistivity between 1 and 5 ohms. Metallization is carried out under vacuum using metals such as aluminum, which is preferred for its low cost, zinc, gold, palladium and cadmium, preferably on a film which has been previously subjected to treatment with a continuous, non-disrupting electric or corona discharge. The bonded sheets are generally prepared by lamination of the two films to each other, with or without interposed adhesives, by causing the films to pass between heated rollers. Adhesives, when used, can be applied to the face of one of the films in the form of an organic solvent solution or aqueous dispersion. Adhesives said to be especially useful are thermoplastic resins such as cellulose esters and ethers, alkyl and acrylic esters, polyamides, polyurethanes, polyesters; or thermosetting resins such as epoxy resins, urea/formaldehyde, phenol/formaldehyde, melamine/formaldehyde; or synthetic rubbers.